Ahem. Excuse me. Demonstrative, so "those". If you're going to put a verb next it'll be "they" and "them" is just the accusative and dative of "they".

Of course you might use "those" directly with a verb, as in "these are nice but those are shite". Then the noun you qualify is implied by the context. The clue is whether is still makes sense if you insert "ones".

Ohmy I'm a bit confused by this too. According to the poster above, those is incorrect as it doesn't have a noun after it. But I'm not sure about "these": presumably the same rule applies. So, you have to say " shall I buy these/ those apples?".

I know my explanation wasn't bullet proof and there were exceptions. So how about this:

"Them " is like "him" and "those" is like "that", so just as you wouldn't say "I heard it from him pedant on the internet" (unless maybe you were Tarzan), you wouldn't say "I heard it from them pedants on the internet". And you would say "I heard it from that pedant" and "I heard it from those pedants".